1. Field of the Invention
This invention relates to an exhaust purification device for an engine, specifically, an exhaust purification device designed such that an additive injected from an additive injection means is supplied to a catalytic device in a state of being diffused in exhaust.
2. Description of the Related Art
An exhaust purification device including an SCR catalyst (selective reduction-type NOx catalyst) is an example of exhaust purification device which purifies exhaust by converting harmful substances in the exhaust into harmless substances using an additive. An exhaust purification device of this type is disclosed in Unexamined Japanese Patent Publication No. 2003-232218 (hereinafter referred to as Patent Document 1), for example.
The exhaust purification device disclosed in Patent Document 1 includes a mixer for agitating exhaust, located upstream of an SCR catalyst in an exhaust pipe of an engine and provided with a plurality of dividing plates, and an injection nozzle for injecting urea aqueous solution, located upstream of the mixer. While the engine is operating, urea aqueous solution is injected from the injection nozzle to the exhaust flow in the exhaust pipe. The urea aqueous solution injected is diffused and atomized in the exhaust by the mixer's agitation, while being hydrolyzed by heat and vapor in the exhaust to form ammonia (NH3). On the SCR catalyst, the ammonia thus formed serves as a reducing agent and causes reduction of NOx (nitrogen oxides) in the exhaust to harmless N2 (nitrogen)
In such exhaust purification device, the amount of urea aqueous solution injected from the injection nozzle should desirably be controlled according to exhaust temperature. Further, when the exhaust temperature has dropped below a lower limit allowing a desired level of ammonia formation, the injection of urea aqueous solution needs to be stopped. Thus, the exhaust purification device of Patent Document 1 includes a temperature sensor disposed between the mixer and the SCR catalyst to control the amount of urea aqueous solution injected from the injection nozzle according the exhaust temperature detected by the temperature sensor.
In the exhaust purification device disclosed in Patent Document 1, however, the temperature sensor is apart from the injection nozzle, so that the urea aqueous solution injection quantity is controlled far upstream of where the exhaust temperature is detected. This hinders the exhaust purification device from controlling the urea aqueous solution injection quantity appropriately. To solve this problem, it is conceivable to dispose the temperature sensor near the injection nozzle. This solution, however, causes the following new problems:
In this case, part of the urea aqueous solution injected from the injection nozzle adheres to the temperature sensor near the injection nozzle, and due to the cooling action of the adhering urea aqueous solution, which is at low temperature, and the latent heat of vaporization of the urea aqueous solution, the temperature detected by the temperature sensor is lower than the actual exhaust temperature. In FIG. 4, how the exhaust temperature is detected by such conventional technology is indicated in a broken line. The graph shows that the value detected by the temperature sensor frequently experiences a temporary drop. As seen in FIG. 4, the timing of the temperature drop almost corresponds to the timing of the urea aqueous solution injection, indicated below the broken line. From this, it can be inferred that the urea aqueous solution adhering to the temperature sensor causes the temporary drop in the value detected by the temperature sensor.
Consequently, in the exhaust purification device with the temperature sensor disposed as described above, the urea aqueous solution injection quantity is controlled inappropriately on the basis of the detected value departing from the true value. For example, although the exhaust is actually at the temperature that allows the reduction of NOx to be caused by injecting the urea aqueous solution, it is determined that the exhaust temperature is below an allowable lower limit, so that the urea aqueous solution injection is stopped. This means missing the timing to reduce NOx, and therefore, failing to make the best use of the NOx-reducing performance of the SCR catalyst to purify the exhaust. It is without saying that later additional injection of the urea aqueous solution in the amount corresponding to the injection stopped only leads to excessive injection of the urea aqueous solution, which leads to useless consumption of urea aqueous solution and ammonia slip from the SCR catalyst.